Rapid Diagnostic Tests Evaluated for Human Plasmodium knowlesi

Until recently only four types of Plasmodium (P. falciparum, P. vivax, P. malariae, and P. ovale) were known to cause malaria in humans, however, a fifth species, P. knowlesi, has been identified as a cause of human malaria in almost all countries in Southeast Asia. P. knowlesi is difficult to diagnose by microscopy because of morphological similarity to P. malariae.

Scientists at the Universiti Malaysia Sarawak, (Malaysia) and their international colleagues diagnosed by microscopy 40 malaria patients, 28 P. knowlesi, 10 P. vivax, and 2 P. falciparum. Patient blood samples were used to determine parasitaemia by microscopy, confirm the Plasmodium species present by polymerase chain reaction (PCR) and evaluate three RDTs using frozen blood samples from 41 P. knowlesi malaria patients.

A large number of RDTs are available for malaria diagnosis, including the three used in this study: OptiMAL-IT (DiaMed; Mississauga, ON, Canada), BinaxNOW Malaria (Alere; Waltham, MA, USA), and Paramax-3 malaria Pf/Pv/Pan (Zephyr Biomedical Systems; Goa, India). The sensitivity of each RDT was calculated with nested PCR results as the reference standard.

OptiMAL-IT was the most sensitive RDT, with a sensitivity of 71% (20/28) for fresh and 73% (30/41) for frozen P. knowlesi samples. However, the test was not specific and P. knowlesi samples cross-reacted with the P. falciparum LDH test reagent in 18 of the 20 fresh samples identified. BinaxNOW Malaria correctly detected non-P. falciparum malaria in P. knowlesi samples but was the least sensitive, detecting only 29% (8/28) of fresh and 24% (10/41) of frozen samples. The Paramax-3 RDT tested positive for P. vivax with PCR-confirmed P. knowlesi samples with sensitivities of 40% (10/25) with fresh and 32% (13/41) with frozen samples. All RDTs correctly identified P. falciparum- and P. vivax-positive controls with parasitaemias above 2,000 parasites/μL blood.

The authors concluded that the sensitivity of detection of P. knowlesi by the three RDTs evaluated is low compared with microscopy. Cross-reactivity is common between P. knowlesi-infected blood and both the P. falciparum-detecting antibody used in the OptiMAL-IT test and the P. vivax-detecting antibody used in the Paramax-3 test. Since not all species of malaria warrant the same level of medical care, misidentification can result in mismanagement, especially when the potentially severe knowlesi malaria is misdiagnosed as vivax malaria. The study was published on February 18, 2014, in the Malaria Journal.

Related Links:
Universiti Malaysia Sarawak
DiaMed
Alere